1. Field of the Invention
The present invention relates to a circular polarizer, and to a liquid crystal display device and a terminal device that use the circular polarizer, and particularly relates to a circular polarizer having excellent wavelength characteristics and excellent viewing angle characteristics when viewed at an angle, and to a liquid crystal display device and a terminal device that use the circular polarizer.
2. Description of the Related Art
Because of their thin profile, light weight, small size, low energy consumption, and other advantages, display devices that use liquid crystals have been widely deployed and used in a range of devices that includes monitors, televisions (TV: Television), and other large terminal devices; notebook-type personal computers, cash dispensers, vending machines, and other mid-sized terminal devices; and personal TVs, PDAs (Personal Digital Assistance: personal information terminal), mobile telephones, mobile gaming devices, and other small terminal devices. In the liquid crystal panel that is the primary component of a liquid crystal display device, information is displayed by using an electric field to control the orientation of liquid crystal molecules, and numerous modes have been proposed according to the combination of the type and initial orientation of the liquid crystal molecules, the direction of the electric field, and other characteristics. Among these modes, the modes most often used in a conventional terminal device include an STN (Super Twisted Nematic) mode using a simple matrix structure, and a TN (Twisted Nematic) mode using an active matrix structure. However, a liquid crystal panel that uses these modes has a narrow range of angles in which contrast ratios can be correctly distinguished, and grayscale inversion occurs outside the optimum viewing position.
This problem of grayscale inversion was relatively insignificant in mobile telephones and other terminal devices when the display content consisted mainly of telephone numbers and other characters. However, with recent technological development, terminal devices have come to display not only text information, but also large amounts of image information. The visibility of images is therefore severely reduced by grayscale inversion. Liquid crystal panels that use a mode having a wide range of angles at which contrast ratio can be correctly distinguished without the occurrence of grayscale inversion are therefore gradually being installed in terminal devices. Liquid crystal panels having this type of mode are referred to generically as wide-viewing-angle liquid crystal panels, and IPS (In-Plane Switching) modes and other horizontal field modes, multi-domain vertical alignment modes, and the like are applied therein.
Among the wide-viewing-angle modes, the multi-domain vertical alignment mode is a scheme that has domains in which the orientation directions compensate for each other in a vertical-alignment-mode liquid crystal panel in which a vertical alignment state exists when a voltage is not applied, and the liquid crystal molecules align parallel to the substrate boundary when a voltage is applied. Specifically, liquid crystal molecules that are oriented in a certain direction are optically compensated for by liquid crystal molecules of another domain that are aligned in a different direction, and the viewing angle is improved.
Although the liquid crystal molecules are oriented at an angle when a voltage is applied in this multi-domain vertical alignment mode, the effects of the angled liquid crystal molecules are optically compensated for, and the viewing angle is improved.
In contrast, in an IPS scheme or other horizontal field mode, the liquid crystal molecules are uniaxially oriented parallel to the substrate, and when a voltage is applied parallel to the substrate, the liquid crystal molecules rotate while maintaining a state parallel to the substrate. Specifically, since the liquid crystal molecules do not stand upright in relation to the substrate even when a voltage is applied, the advantage of a wide viewing angle is gained in principle.
In a display device that uses liquid crystals, the liquid crystal molecules as such do not emit light, and some type of light must therefore be used in order for the display to be visible. These liquid crystal display devices can be generally classified as transmissive, reflective, or semi-transmissive (using transmitted light and reflected light jointly) according to the type of light source used. Energy consumption can be reduced in the reflective type, since external light can be utilized in the display device, but contrast ratio and other aspects of display performance are inferior compared to the transmissive type. Therefore, transmissive and semi-transmissive liquid crystal display devices are currently in the mainstream. In transmissive and semi-transmissive liquid crystal display devices, a light source is installed on the back surface of a liquid crystal panel, and a display is created using the light emitted by the light source. In particular, medium-sized liquid crystal display devices are carried by a user and used under various conditions. Therefore, semi-transmissive liquid crystal display devices that have high visibility in any situation are used as such medium-sized devices as a result of the fact that the reflective display is visible in bright locations, and the transmissive display is visible in dark locations.
An ECB (Electrically Controlled Birefringence) mode or the aforementioned multi-domain vertical alignment mode having characteristics of high resolution and wide viewing angle has been used in the past in liquid crystal panels used in these semi-transmissive liquid crystal display devices.
FIG. 1 is a sectional view showing the vertical-alignment-mode semi-transmissive liquid crystal panel used in the first conventional liquid crystal display device described in AsiaDisplay/IDW01, p. 134. As shown in FIG. 1, in the vertical-alignment-mode semi-transmissive liquid crystal display device that is the first conventional example, a backlight 4007, a lower polarizer 4006, a lower λ/4 plate 4005, a liquid crystal layer 4003, an upper λ/4 plate 4002, and an upper polarizer 4001 are layered in sequence from the back side, and a reflecting plate 4004 is formed under the liquid crystal layer 4003 of the reflective display region. Since liquid crystals having negative dielectric anisotropy are vertically oriented in the liquid crystal layer, there is no refractive index anisotropy in the plane of the display when a voltage is not applied, and the liquid crystal layer is isotropic. The upper λ/4 plate 4002 and the lower λ/4 plate 4005 are also arranged so that the slow axes thereof are orthogonal to each other. An optical sheet in which a λ/4 plate and a polarizer that emits linearly polarized light are placed together so that the absorption axis of the polarizer and the low axis of the λ/4 plate are at a 45 degree angle in the manner of the present conventional example has the effect of emitting circularly polarized light. A polarizer that generates circularly polarized light in this manner is generally referred to as a circular polarizer, and is distinguished from a polarizer that emits linearly polarized light.
In the first conventional vertical-alignment-mode semi-transmissive liquid crystal panel thus configured as described in AsiaDisplay/IDW01, p. 134, light that passes through the upper polarizer 4001 enters the upper λ/4 plate 4002 as linearly polarized light, and is emitted as counterclockwise circular polarized light in the reflective display region in the off state in which a voltage is not applied. The light then enters the liquid crystal layer 4003, but since the liquid crystal layer does not have refractive index anisotropy in the plane of the display as previously mentioned, there is no change in the polarization state. Consequently, the light enters the reflecting plate 4004 without modification as counterclockwise circularly polarized light, is converted to clockwise circularly polarized light upon being reflected by the reflecting plate 4004, and enters the liquid crystal layer 4003. The light that passes as clockwise circularly polarized light through the liquid crystal layer 4003 without modification reenters the upper λ/4 plate 4002, and is converted to linearly polarized light, but since the light enters in a state of circular polarization in the opposite direction from when the light entered, the emitted light is linearly polarized light that is orthogonal to the incident light. The light that enters the upper polarizer 4001 is therefore absorbed by the polarizer. Specifically, the display turns black when a voltage is not applied in the reflective display part. However, when a voltage is applied, since the liquid crystal that was vertically aligned is no longer upright, and birefringence occurs in the display plane, the polarization state changes so that light is emitted, and the display turns white. Specifically, a normally black reflective display is created.
In the transmissive region in the off state in which a voltage is not applied, light emitted from the backlight 4007 that enters the lower polarizer 4006 becomes linearly polarized light, enters the lower λ/4 plate 4005, and enters the liquid crystal layer 4003 as clockwise circularly polarized light. As described above, the liquid crystal layer 4003 to which a voltage is not applied does not have optical anisotropy in the display plane, and the incident clockwise circularly polarized light is therefore emitted without modification from the liquid crystal layer 4003, and enters the upper λ/4 plate 4002. The light that enters the upper λ/4 plate 4002 is converted to linearly polarized light, but is absorbed by the upper polarizer 4001. Specifically, the display turns black when a voltage is not applied in the transmissive display part. However, when a voltage is applied, since the liquid crystal that was vertically aligned is no longer upright, and birefringence occurs in the display plane, the polarization state changes so that light is emitted, and the display turns white. Specifically, a normally black reflective display is created.
FIG. 2 is a schematic sectional diagram showing the vertical-alignment-mode semi-transmissive liquid crystal panel used in the second conventional liquid crystal display device described in Japanese Laid-open Patent Application No. 2000-035570. The circular polarizer in the first conventional liquid crystal display device was composed of a polarizer and a λ/4 plate, whereas the circular polarizer in the present conventional example is composed of a polarizer, a λ/2 plate, and a λ/4 plate. As shown in FIG. 2, in the vertical-alignment-mode semi-transmissive liquid crystal display device as the second conventional example, a polarizer 2009, a λ/2 plate 2012, a λ/4 plate 2010, a substrate 2001, a reflection electrode 2003 and a transmission electrode 2008, an LC layer (vertical alignment) 2005, an opposing electrode 2004, a substrate 2002, a λ/4 plate 2007, a λ/2 plate 2011, and a polarizer 2006 are layered in sequence from the back side. The slow axis of the λ/2 plate 2012 and the slow axis of the λ/2 plate 2011, and the slow axis of the λ/4 plate 2010 and the slow axis of the λ/4 plate 2007 are arranged orthogonally to each other, respectively. The LC layer is a liquid crystal layer.
In the first conventional liquid crystal display device, the circular polarizer was configured as a wavelength plate having a polarizer and a λ/4 plate. A wavelength plate is generally fabricated from a polymer film, wherein the refractive index anisotropy of the polymer film increases the shorter the wavelength, and decreases the longer the wavelength. Therefore, when the wavelength plate is set so as to be a λ/4 plate at 550 nm, for example, the wavelength plate does not function adequately as a λ/4 plate at wavelengths other than those near 550 nm, due to the wavelength dependency of the refractive index anisotropy of the λ/4 plate, light leakage occurs at wavelengths other than those near 550 nm in the reflective mode of a dark display, and an adequate black level is not obtained.
In the second conventional liquid crystal display device, since the circular polarizer is composed of a polarizer, a λ/2 plate, and a λ/4 plate, the wavelength dependency of the refractive index anisotropy that occurs when linearly polarized light is converted to circularly polarized light is cancelled out to a certain degree. The light can thereby be converted to circularly polarized light in the reflective mode in a state in which there is minimal fluctuation in the polarization state in a wide wavelength band of the visible light region. The contrast ratio of the reflective mode and the coloration of a dark display in the reflective mode can therefore be improved. A circular polarizer that is capable of circularly polarizing light in a state in which there is minimal fluctuation of the polarization state in a wide wavelength band is referred to as a broadband circular polarizer.
In the second conventional liquid crystal display device, the broadband circular polarizer was composed of a polarizer, a λ/2 plate, and a λ/4 plate, but according to Japanese Laid-open Patent Application No. 2000-35570, a broadband circular polarizer can be formed using a different configuration. For example, a polarizer and three λ/2 plates may be used, or a polarizer, two λ/2 plates, and a λ/4 plate may be used.
A circular polarizer is used not only in vertical-alignment-mode semi-transmissive liquid crystal panels, but also in multi-domain vertical-alignment-mode transmissive liquid crystal panels. When a linear polarizer is used in a multi-domain vertical alignment mode, regions in which liquid crystals lie down parallel or orthogonal to the absorption axis of the linear polarizer do not contribute to the transmittance. However, when a circular polarizer is used in a multi-domain vertical alignment mode, the liquid crystals contribute to the transmittance regardless of the direction in which the liquid crystals lie down, and the transmittance is increased.
However, the conventional techniques described above have such problems as the following.
Specifically, compared to a case in which a linear polarizer is used, a liquid crystal display device that uses the conventional circular polarizer has inferior viewing angle characteristics.
The cause of this problem will be described using an example in which the circular polarizer is composed of a polarizer and a λ/4 plate. The λ/4 plate in the circular polarizer is designed so as to act as a λ/4 plate with respect to light that enters from the direction normal to the surface of the λ/4 plate. Specifically, the λ/4 plate is designed so that the retardation Re of the in-plane direction is one-fourth of the wavelength. The retardation Re is determined by the thickness d of the λ/4 plate and the difference between the in-plane refractive indices nx, ny in the principal axis direction. As for light that is at an angle from the normal line, i.e., when the display is viewed from an angle, the retardation is not solely the Re determined by nx and ny, but is also affected by the refractive index nz of the principal axis in the thickness direction, and the increased optical path length due to the angle in relation to the normal line. Therefore, when the display is viewed at an angle, the λ/4 plate functions as a retardation plate that differs from the original λ/4 plate, and the combination of the polarizer and the λ/4 plate functions as an elliptical polarizer rather than a circular polarizer. Furthermore, a circular polarizer composed of one polarizer and a plurality of in-plane retardation plates is more significantly affected by the difference of the retardation when viewed at an angle than a circular polarizer that is composed of one polarizer and one in-plane retardation plate having retardation in the in-plane direction.
Since a linear polarizer is used in the transmissive liquid crystal display device, the viewing angle performance is superior to the viewing angle performance obtained when a circular polarizer is used. It was therefore impossible to enhance the viewing angle performance of a common transmissive liquid crystal display device beyond that of the transmissive display of the conventional semi-transmissive liquid crystal display device.
Furthermore, since the conventional broadband circular polarizer also had unsatisfactory viewing angle characteristics, the viewing angle performance of the reflective display of the semi-transmissive liquid crystal display device could not be enhanced beyond the viewing angle performance of the common transmissive liquid crystal display device.